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Principle of relativity for quantum theory

In
a generic quantum experiment we have a given set of devices analyzing some
physical property of a system. To each device involved in the experiment we
associate a set of random outcomes corresponding to the possible values of the
variable analyzed by the device. Devices have apertures that permit physical
systems to pass through them. Each aperture is labelled as "input" or
"output" depending on whether it is assumed that the aperture lets
the system go inside or outside the device. Assuming a particular input/output
structure for the devices involved in a generic experiment is equivalent to
assume a particular causal structure for the space-time events constituted by
the outcomes happening on devices. The joint probability distribution of these
outcomes is usually predicted assuming an absolutely defined input/output
structure of devices. This means that all observers of the experiment agree on
whether an aperture is labelled as "input" or "output". In
this talk we show that the mathematical formalism of quantum theory permits to
predict the joint probability distribution of outcomes in a generic experiment
in such a way that the input/output structure is indeed relative to an
observer. This means that two observers of the same experiment can predict the
joint probability distribution of outcomes assuming different input/output
labels for the apertures. Since input/output structure is the causal structure
of the space-time events constituting the outcomes involved in the experiment
we conclude that in quantum theory, the causal structure of events may not be
regarded as absolute but rather as relative to the observer. We finally point
out that properly extending this concept to the cosmological domain could shed
light on the problem of dark energy.